We have worked through the intricacies of dietary fats in our last blog posts — discussing the role of the macronutrient for our health, before understanding the different types of fats and their effect on our well-being. This naturally raises another question. We know dietary fats play an important role for our health and well-being — but what about their role in exercise performance? There are different diets used in the world of sports, such as the ketogenic diet or other high-fat diets. How do these fat-rich diets affect exercise performance?
Energy density vs energy rate
To understand why some athletes adopt high-fat diets, we should understand how our bodies produce energy used for exercise. From an energetic perspective, dietary fats are more energy-dense, providing more energy per gram than any other macronutrient (Jeukendrup & Gleeson, 2023). However, the oxygen required to produce energy from dietary fats is higher than when using carbohydrates (Kozjek Schwietert et al., 2026).
Therefore, while overall energy production may be higher when consuming dietary fats, the rate at which energy is produced is lower than when carbohydrates are used. With this in mind, it becomes clearer that the discussion of fats and sports performance focuses on endurance sports — intermittent sports, team sports, and resistance exercise are repeated bouts of very high-intensity efforts where carbohydrates can provide energy at a faster rate. From this starting point, we can get a deeper look at dietary fats and performance.
Fat adaptation: what you gain, what you lose
With increasing exercise intensity, the body relies more and more on carbohydrates as its preferred fuel source (Van Loon et al., 2001). This can be seen in the recent “carbohydrate revolution” which has shaped endurance sports in the last decade — looking at you, record cycling performances. Still, an athlete who is adapted to a high fat intake may utilise more fat during exercise and spare carbohydrate stores for periods of high-intensity effort.
However, there is a compromise to be made. Adaptation to a high-fat diet increases the body’s ability to consume and use dietary fats for energy, but it decreases the use of carbohydrates (Burke et al., 2000). The issue with such a compromise is that many endurance events have such high intensity demands that carbohydrates are simply the better-suited fuel:
- Marathon running is performed at a relatively high percentage of VO2max, so the body relies heavily on carbohydrates during such races (Jones et al., 2021).
- Cycling races may last up to 7 hours with a large reliance on carbohydrate at critical points (Valenzuela et al., 2026).
- In elite race walkers, fat adaptation showed significantly slower race times (~5% over a 10 km race) compared to a high-carbohydrate diet following a training camp. In fact, the training benefits — such as increased VO2max — were counteracted by the fat adaptation, which reduced exercise economy (Burke et al., 2017).
Therefore, in events where intensity is elevated, high-fat diets seem to impair exercise performance. There may be benefits to such a strategy in ultra-endurance, multi-day events, where the lower intensity can be sustained with fat as the major fuel source. Additionally, the energy density of fats may help with energy balance during long, demanding days of exercise.
The bottom line
Being adapted to high-fat diets does not translate to better performance in most events. While dietary fats are essential for our health and well-being, most athletes still rely on carbohydrates during their events due to the high-intensity demands of most sports. As such, while some ultra-endurance athletes may benefit from high-fat adaptation, for the majority of athletes dietary fat should be a tool — part of a balanced, varied diet for well-being — while the focus for high-intensity performance stays on carbohydrates.
📚 Part of our macronutrient series. Related: Why we rely on carbs during exercise · Different types of fats · The forgotten macronutrient
References
Burke, L. M., Angus, D. J., Cox, G. R., Cummings, N. K., Febbraio, M. A., Gawthorn, K., Hawley, J. A., Minehan, M., Martin, D. T., & Hargreaves, M. (2000). Effect of fat adaptation and carbohydrate restoration on metabolism and performance during prolonged cycling. Journal of Applied Physiology, 89(6), 2413–2421. https://doi.org/10.1152/jappl.2000.89.6.2413
Burke, L. M., Ross, M. L., Garvican‐Lewis, L. A., Welvaert, M., Heikura, I. A., Forbes, S. G., Mirtschin, J. G., Cato, L. E., Strobel, N., Sharma, A. P., & Hawley, J. A. (2017). Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. The Journal of Physiology, 595(9), 2785–2807. https://doi.org/10.1113/JP273230
Jeukendrup, A., & Gleeson, M. (2023). Sport nutrition (4th ed.). Human Kinetics.
Jones, A. M., Kirby, B. S., Clark, I. E., Rice, H. M., Fulkerson, E., Wylie, L. J., Wilkerson, D. P., Vanhatalo, A., & Wilkins, B. W. (2021). Physiological demands of running at 2-hour marathon race pace. Journal of Applied Physiology, 130(2), 369–379. https://doi.org/10.1152/japplphysiol.00647.2020
Kozjek Schwietert, A. K., Knap, K., Tonin, G., Rotovnik Kozjek, N., & Gleeson, M. (2026). Energy, carbohydrates and fats in clinical sports nutrition. Clinical Nutrition ESPEN, 71, 102900. https://doi.org/10.1016/j.clnesp.2025.102900
Valenzuela, P. L., Leo, P., Mateo-March, M., Gallo, G., Seiler, S., & Mujika, I. (2026). UCI Sports Nutrition Project: the science of successful cycling performance. International Journal of Sport Nutrition and Exercise Metabolism, 36(3), 174–186. https://doi.org/10.1123/ijsnem.2025-0157
Van Loon, L. J. C., Greenhaff, P. L., Constantin‐Teodosiu, D., Saris, W. H. M., & Wagenmakers, A. J. M. (2001). The effects of increasing exercise intensity on muscle fuel utilisation in humans. The Journal of Physiology, 536(1), 295–304. https://doi.org/10.1111/j.1469-7793.2001.00295.x